Expired Gas Collection for Dead Space Calculations: A Comparison of Two Methods

The ratio of physiological dead space to tidal volume (V D /V T ) and carbon dioxide production (VCO 2 ) are often followed in assessing the physiological status of patients receiving mechanical ventilation. We compared two methods of determining V D /V T , one that corrects for compressible circuit volume and one that eliminates compressible volume from the collected sample. In 11 patients receiving assisted mechanical ventilation, agreement between the two determinations was excellent. Collection of expired gas using the second method described allows simultaneous determination of both V D /V T and VCO 2 and is not affected by the mode of ventilation.